In the event of a failure in the operation of the elevator, and in particular in the event of a failure of the public or industrial electricity mains network powering the motor, or in the event of a failure of the motor or the converter, the elevator cabin can remain blocked between two floors at a level that does not allow the people inside the cabin to be evacuated.
Under such circumstances, an operator is called on to act to release the brake manually. Under the effect of being out of balance with the counterweight, the cabin can then move downwards or upwards in order to reach a door allowing it to be evacuated.
During this operation the operator controlling the brake must take care that the cabin does not move too quickly by visually estimating its speed, so as to ensure that the people it is carrying are safe and so as to avoid damaging the elevator.
Having recourse to a visual estimate of the speed is not very satisfactory in terms of safety and makes the action difficult to perform.
International patent application WO 01/46056 discloses a safety device in which the electricity delivered by the motor under the effect of the cabin moving is dissipated in an electrical resistance, thereby tending to brake the rotor. Such a device reduces acceleration of the cabin, but it does not eliminate the need to take manual action on the brake. Furthermore, that device is ineffective in the event of a failure in the electrical circuit connecting the motor to the resistance in which energy is dissipated.